Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging”, can be performed by several methods.
In conventional oil well wireline logging, a probe or “sonde” housing formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The upper end of the sonde is attached to a conductive wireline that suspends the sonde in the borehole. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
An alternative method of logging is the collection of data during the drilling process. Collecting and processing data during the drilling process eliminates the necessity of removing or tripping the drilling assembly to insert a wireline logging tool. It consequently allows the driller to make accurate modifications or corrections as needed to optimize performance while minimizing down time. Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as “measurement-while-drilling” techniques, or “MWD”. Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as “logging while drilling” techniques, or “LWD”. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
Sensors or transducers typically are located at the lower end of the drill string in LWD systems. While drilling is in progress these sensors continuously or intermittently monitor predetermined drilling parameters and formation data and transmit the information to a surface detector by some form of telemetry. Typically, the downhole sensors employed in LWD applications are positioned in a cylindrical drill collar that is positioned close to the drill bit. The LWD system then employs a system of telemetry in which the data acquired by the sensors is transmitted to a receiver located on the surface.
Yet another method of gathering downhole data is seismic imaging. One method of seismic imaging involves stringing hundreds of listening devices, or geophones, along the length of a borehole in the Earth near a location where a characteristic picture of the underground formations is desired. Geophones include particle velocity detectors for measuring both compressional and shear waves directly. Geophones typically provide three-component velocity measurement, and consequently can be used to determine the direction of arrival of incident elastic waves. Once these geophones are strategically placed, a seismic disturbance is created which creates traveling waves through the Earth's crust. As these traveling waves encounter boundaries of strata having varying densities, portions of the traveling wave reflect on their way to the seismic array. These varying density stratas may include changes in strata components as well as varying densities encountered at boundaries of hydrocarbon reservoirs. By measuring the propagation time, amplitude and direction of reflected waves as they reach the array, a three-dimensional representation of the formations lying below the surface of the Earth can be constructed (“3D seismic imaging”).
After a particular hydrocarbon formation is found, the need for information is not alleviated. Once a hydrocarbon reservoir is tapped, the goal becomes removing as much of the hydrocarbons from the reservoir as possible. Here again, the more information one has about the locations of hydrocarbons within the reservoir over the course of time, the more likely the hydrocarbons contained in the reservoir can be fully extracted at the lowest possible cost. Having multiple three-dimensional seismic representations of conditions below the surface over time is typically referred to as four-dimensional (4D) seismic imaging. In early implementations, four-dimensional seismic was created by performing multiple three-dimensional seismic images of the strata or hydrocarbon reservoir in question. The time period for taking readings to determine migration patterns of the hydrocarbons may be as long as years. That is, a single three-dimensional seismic reading may be taken once a year over the course of several years to obtain the four-dimensional seismic image.
One fundamental requirement of both 3D and 4D seismic imaging is measuring the arrival time of reflected waves at one location relative to arrival of reflected waves at another location. To accomplish this task, large quantities of information must be recorded, substantially simultaneously, to correlate the arrival time of the various reflected waves.
Information is the key to being profitable in the oil and gas industry. The more information that can be gathered, the higher the efficiency of the drilling and extraction operations can be made. To this end, new and more sophisticated sensor arrangements are routinely created and placed in the borehole, so much so that the information carrying capacity of traditional telemetry techniques are becoming inadequate. For these reasons it would be desirable to have a communication technique that can support high speed communications between downhole sensors and a surface installation.